1. Field of the Invention
This invention relates to a method of growing semiconductor crystal for the crystallization of an amorphous silicon layer such as a SOI substrate or a SOS substrate.
2. Description of the Related Art
Load type memory cells formed from polycrystalline silicon are used for a static RAM (random access memory) (hereinafter referred to as SRAM) of the high resistance load type. However, a SRAM of the high resistance load type is difficult to sufficiently assure an operation margin, reliability, a stand-by current and so forth. In order to solve these problems, a SRAM of the layered type has been proposed wherein thin film transistors formed from thin films of amorphous silicon or polycrystalline silicon are employed as load elements. Thin film transistors are employed also for liquid crystal display panels. Polycrystalline silicon thin films are commonly employed for thin film transistors for which high performances for an on-current characteristic, a sub-threshold characteristic, an on/off current ratio and so forth are required. Thus, efforts have been and are being made to achieve enhancement of the characteristics by increasing the size of polycrystalline silicon grains and decreasing the trap density in a Polycrystalline silicon thin film.
Various methods of forming polycrystalline silicon have been proposed including an ordinary chemical vapor phase growing method and a random solid phase growing method. Particularly the random solid phase growing method allows formation of a polycrystalline silicon layer wherein the crystalline grains have a large diameter of 1 .mu.m or more.
Also a method of forming thin film transistors in a selectively formed single crystal region has been proposed.
In order to improve the problems described above where a polycrystalline silicon film is employed, a further method has been proposed in recent years wherein the dose of implantation of Si.sup.+ for the conversion into the amorphous phase is reduced at a predetermined position, at which a core for the crystallization is to be formed. The method is disclosed in H. Kumomi et al., "Control of Grain-Location in Solid State Crystallization of Si", Extended Abstracts of the 22nd (1990 International) Conference on Solid State Devices and Materials, Sendai, 1990, pp. 1159-1160 and also in Japanese Patent Laid-Open Application No. 3-125422.
The method disclosed in Japanese Patent Laid-Open Application No. 3-125422 is described subsequently with reference to FIGS. 3(1) to 3(3). In particular, ions of silicon (Si.sup.+) are first implanted at a low dose into a polycrystalline silicon layer 52 on a SiO.sub.2 layer 51 as shown in FIG. 3(1). Then, a resist mask 53 is formed on an upper face of the polycrystalline silicon layer 52 as shown in FIG. 3(2) using the lithography technique, and ions of Si.sup.+ are selectively implanted at a high dose into portions of the polycrystalline silicon layer 52 at which it is not covered with the resist mask 53. Subsequently, the resist mask 53 is removed, and crystals of silicon are grown at a high dosage by the low temperature solid phase growing method at locations around the regions in which ions have not been implanted, thereby forming single crystal silicon regions 54.
With the method of forming a polycrystalline silicon film by the ordinary chemical vapor phase growing method, however, if it is intended to form crystal grains of a large size, then it is difficult to form a polycrystalline silicon film which is superior in uniformity of the film quality and has a high mobility with low leakage.
Meanwhile, with the random solid phase growing method, since it is difficult to selectively grow crystal, it sometimes occurs that a channel of a transistor extends to a grain boundary. If this actually occurs, then a dispersion occurs the leakage current or threshold voltage, which deteriorates the reliability of the transistor.
On the other hand, with the method of selectively forming a single crystal silicon region, also the problem of pollution of resist is involved and a low dose region is non-uniform. Further, that the resist is left locally is unstable and has with holes.
Further, also a method of irradiating an argon ion laser beam to achieve crystallization, a SIMOX method, a zone melt method and so forth have been proposed as methods of forming a SOI substrate. However, any of the methods is low in repeatability and also in throughput.
Meanwhile, with a method wherein a laser beam is selectively irradiated is disclosed in Japanese Patent Laid-Open Application No. 3-285720, it is difficult to assure the uniformity of an excimer laser beam.